The Air Force has renewed a long-smoldering controversy within the Pentagon over whether the United States should prevent the Soviet Union from being able to read the telemetry from military missile tests.

Air Force officials want to encode the signals from U.S. missile tests in order to keep the Soviets from being able to learn technical secrets. But defense Secretary Harold Brown is resisting these demands, as have his predecessors in the Pentagon.

Pentagon sources indicate that the Soviets, at least on some SS-20 and other test missile flights, have encoded the telemetry to prevent the United States from monitoring the flights.

A central issue in the dispute is whether it is contrary to the letter or, at least the spirit, of the SALT I agreement to screen missile signals. The agreement stipulated that "each party undertakes not to interfere with the national technical means of verification of the other parties."

Brown is understood to have taken the position that he wants to persuade the Soviets that it is in the mutual interest of the two superpowers to monitor each other's tests - in effect, read each other's electronic mail in the strategic weapons area.

The telemetry in the tests at issue provides information on the status of each country's missile program, such data as range, accuracy and likely payload of the latest strategic rockets.

Some U.S. sources suggest that the Soviets may not regard the encoding of missile telementry as a violation of the SALT I agreement. On the other hand, the U.S. Arms Control and Disarmament Agency regards such signals as part of the "national technical means of verification" which the treaty requires be kept open to all sides, according to its spokesman, Thomas A. Halstead.

There have been no official U.S. contentions, however, that the Soviet Union has been in violation of the agreement.

Air Force officials argue that the United States is on the losing end of the proposition - with its telemetry open to Soviet monitors while the Russians encode their own.

The issue was revived recently by former Defense Secretary Melvin R. Laird in the December issue of Reader's Digest. He is senior counselor for national and international affairs for the magazine.

He wrote that the Soviets "in further contempt of SALT I have sought to hide the capabilities of another new missile, the SS-20. The Russians are aware that ordinarily we, too, can receive and analyze the telementry from their tests, so they encrypted SS-20 telementry to make it incomprehensible to us."

However, Laird went on, when the United States finally deciphered the SS-20 telemetry, analysts concuded that the missile had been tested with 2,000 pounds of ballast. Were that amount of ballast to be replaced with fuel, the SS-20 would become an intercontinental missile capable of reaching North America.

During his tenure as Secretary of Defense, Laird too rejected the Air Force pleas to encode American missile telemetry on grounds that it would violate the SALT I verification provision.